The use of a quartz oscillator in a sensor has been described in U.S. Pat. No. 6,223,589. U.S. Pat. No. 5,741,961 also discloses a quartz resonator for use in an engine oil sensor. Yet another piezoelectric sensor for engine oil is disclosed in Hammond, et al., “An Acoustic Automotive Engine Oil Quality Sensor”, Proceedings of the 1997 IEEE International Frequency Control Symposium, IEEE Catalog No. 97CH36016, pp. 72–80, May 28–30, 1997.
An improved system for measuring characteristics of fluids using mechanical resonators is disclosed in commonly-owned U.S. Pat. Nos. 6,401,519; 6,393,895; 6,336,353; and 6,182,499.
The use of acoustic sensors has been addressed in applications such as viscosity measurement in “Acoustic Wave Microsensors,” J. W. Grate, et at, Anal. Chem. 65, 940A–948A (1993)); “Viscosity and Density Sensing with Ultrasonic Plate Waves”, B. A. Martin, S. W. Wenzel, and R. M. White, Sensors and Actuators, A21–A23 (1990), 704–708; “Preparation of chemically etched piezoelectric resonators for density meters and viscorneters”, S. Trolier, Q. C. Xu, R. E. Newnham, Mat.Res. Bull. 22, 1267–74 (1987); “On-line Sensor for Density and Viscosity Measurement of a Liquid or Slurry for Process Control in the Food Industry”, Margaret S. Greenwood, Ph. D. James R. Skorpik, Judith Ann Bamberger, P. E. Sixth Conference on Food Engineering, 1999 AIChE Annual Meeting, Dallas, Texas; U.S. Pat. Nos. 5,708,191; 5,886,250; 6,082,180; 6,082,181; and 6,311,549; and “Micromachined viscosity sensor for real-time polymerization monitoring”, O. Brand, J. M. English, S. A. Bidstrup, M. G. Allen, Transducers '97, 121–124 (1997).
Notwithstanding the above, there remains a need in the art for alternative or improved sensors for analyzing fluids used in machines (such as those in automotive systems), particularly for measuring changes in fluid amounts, changes in fluid quality or combinations thereof.